The present invention relates to a temperature sensing device and more particularly, to a method of manufacturing a resistance thermometer mainly composed of platinum and the resistance thermometer produced by said method.
Heretofore, platinum has been widely employed as a material for temperature sensors because, due to its favorable chemical stability, platinum with a high purity may be readily obtained, together with platinum having a large temperature dependency in its electrical resistance. In connection with the above, a temperature sensor in which a very fine platinum wire which is spirally wound around an insulator, such as mica or the like, is inserted in a protective tube, has been widely applied to actual resistance thermometers, and standards for these are specifically set forth in JIS (Japanese Industrial Standards) C-1604, DIN 43760 and IEC Pub. 751.
However, the known platinum resistance thermometer although these thermometers do provide a high degree of accuracy, do have the following various disadvantages:
(i) Low mechanical strength PA1 (ii) Troublesome procedures for manufacture PA1 (iii) Large size PA1 (iv) High cost PA1 (ii) High mechanical strength. PA1 (iii) Scattering in characteristics can be reduced through a wafer treatment, and thus, the thermometer is suitable for mass production, with a consequent reduction in cost.
In order to overcome the disadvantages as described above, resistance thermometers employing thick films or thin films of platinum have recently been developed, some of which are commercially available. However, since thick film platinum resistance thermometers are based on the screen printing technique, it is difficult in the manufacture of these thermometers to obtain a very fine pattern at a size smaller than 100 .mu.m, with a large scattering. Meanwhile, the thin film platinum resistance thermometer generally has the following advantages:
(i) Since a very fine pattern is readily formed, the thin film resistance thermometer can be made compact in size, while a high sensitivity thereof may be achieved through high resistance.
Commonly, as a method of manufacturing a resistance thermometer with the use of the aforementioned thin film platinum, there have been adopted the steps of causing a thin film platinum with film thicknesses in the order of several thousands angstroms, to adhere on an insulative substrate through the employment of a vacuum deposition process, a sputtering process, etc., forming the platinum thin film into a fine pattern by a wet etching process, sputter etching process or the like, and heat-treating the platinum thin film thus processed in an atmosphere at high temperatures of 800.degree. to 1,400.degree. C., with subsequent resistance adjustments by trimming, formation into a chip, and leading out lead wires, etc., to obtain the resultant resistance thermometer.
Of the various methods for forming the platinum thin film, in the case in which the sputtering process is adopted, it is common practice to employ an inert gas, such as argon or the like, as the sputtering gas. However, the temperature coefficient of resistance (referred to as TCR hereinafter) of the platinum thin film produced in the above method is considerably smaller when compared with the TCR of the bulk material, and for the above reasons, there may be considered the following factors. For example, in connection with the characteristics of the material for the thin film as referred to above, size effects and structural defects are characteristics which affect the physical properties to a large extent. The size effects mean influences which take place in all transport phenomenon of electrons resulting from reduction in the mean free path of electrons in terms of effectiveness due to nonelastic struggling or dispersion of the electrons within the thin film, and such influences become conspicuous particularly when the film thickness is generally of the same degree or less than the mean free path of the electrons. Meanwhile, since the process of forming the thin film involves a rapid cohesion from gaseous phase to solid phase in a space where gas molecules or ions, which are unrelated to the thin film substance are present, all structural defects which are inherent in crystals such as holes, interstitial atoms, various dislocations, stacking fault, grain boundary, etc., are introduced into the thin film, while different types of atoms and molecules are mixed as impurities, resulting in a scattering of the electrons.
Due to the influences by the above phenomenon, a characteristic in which the specific resistivity of the thin film becomes large as compared with the bulk platinum material results, and accordingly, the TCR of the platinum thin film becomes lower than the TCR of the bulk platinum material, with a consequent lowering of the sensitivity of the platinum thin film as a resistance thermometer. Therefore, it has been conventionally required to develop a manufacturing method capable of obtaining a platinum thin film having a large TCR, without any defects as described above.
Meanwhile, alumina, sapphire, silicone, glass, etc., are employed as the substrate of the platinum thin film, and respectively, have merits and demerits as described below.
Namely, the alumina substrate is inexpensive and heat-resistant, with superior adhesion to platinum, but its rough surface presents a problem in the formation of a very fine pattern. Although the surface may be smoothed, if a surface polishing is applied, the polishing of the alumina substrate with a large hardness, results in an extreme cost increase in the substrate material. The sapphire substrate is superior in heat-resistance, adhesion with respect to platinum and smoothness on the surface, but has such disadvantages in that it is very expensive, and moreover, it is difficult to cut into very small chips, etc. Although silicone substrate is advantageous in that it is comparatively cheap, with a good smoothness, and can be easily processed, there is the drawback in that an alloy is formed with respect to the platinum thin film when a heat-treatment is effected at high temperatures, thus also resulting in a problem related to sensor characteristics. The glass substrate is inexpensive, but has the problems in that it is not provided with a sufficient adhesion with respect to the platinum, and that it is inferior in the heat-resistance.
As described earlier, in the method of manufacturing a resistance thermometer with the use of a thin film platinum, it is common practice to adopt the steps of causing the thin film platinum having film thicknesses, in the order of several thousand angstroms, to adhere onto the insulative substrate through the employment of the vacuum deposition process, sputtering process, etc., forming the platinum thin film into a fine pattern by the wet etching process, sputter etching process or the like, and heat-treating the platinum thin film thus processed in an atmosphere at high temperatures of 800.degree. to 1,400.degree. C., with subsequent resistance adjustments by trimming, leading out lead wires and formation of a protective coating, etc.
For the resistance adjustments as described above, in many cases, there has recently been employed a method in which part of the resistance adjusting pattern of the resistance film is cut off by a laser beam, and in this cut-off processing, the resistance adjusting pattern is cut off in several stages for adjustments through the addition of respectively different resistance values so as to finally obtain the necessary resistance value. For example, the adjustment is effected in such an order that it is made at 5% in the first stage, 1% in the second stage, and 0.2% in the third stage with respect to the total resistance value of the resistance film. Since very complicated adjustments of the resistance value are required as described above, there are involved such problems, such as the yield in production is lowered due to mistakes with respect to the adjusting values by workers or many working hours are required for the complicated work.
Moreover, with respect to a flow rate or flow velocity measuring method capable of measuring the temperature and flow velocity of a liquid through utilization of the resistance thermometer as in the present invention, there have been various conventionally known modes, and in the case where measurements are taken the through utilization of a variation in the amount of heat to be transmitted from a heat generating resistor to a fluid, such amount of heat conducted to the fluid also varies with respect to the temperature variation of the fluid. In the conventional measuring method, to compensate for such an inconvenience, there is provided a resistance thermometer for measuring the fluid temperature, and the resistance thermometer and heat generating resistor are connected in a bridge circuit so that a temperature difference therebetween is held constant, while the output from the bridge circuit to be fed back is amplified for the compensation of the influence by the fluid temperature. However, in the above known practice, for the resistance thermometer to be able to accurately measure the temperature of the fluid, it is necessary to space the resistance thermometer and the heat generating resistor to avoid the influence of the heat by the heat generating resistor, thus introducing drawbacks, which would result in the size of the measuring unit to become large, with a reduction in mass-productivity. Meanwhile, it is required to render the temperature coefficient of the resistance thermometer to coincide with that of the heat generating resistor, but the perfect coincidence therebetween is extremely difficult, thus resulting in the cause of measuring errors.